1. Field of the Invention
The present invention relates to an image recording medium which records image information by storing electric charges generated in a recording-side photoconductive layer in response to irradiation with a recording electromagnetic wave carrying the image information.
2. Description of the Related Art
U.S. Pat. No. 6,268,614 issued to the present inventor (Shinji Imai) and corresponding to Japanese Patent Applications Nos. 10-215378 and 10-232824 (which are laid open as Japanese Unexamined Patent Publication No. 2000-105297) discloses information related to the present invention.
Conventionally, radiographic-image recording mediums which record radiographic images by storing in a charge storage region electric charges the amounts of which respectively correspond to doses of radiation such as an X-ray are used in many applications such as medical radiography, and various types of radiographic-image recording mediums basically operating as above have been proposed. In each of the above radiographic-image recording mediums, information on the radiographic image recorded in the radiographic-image recording medium can be read out by scanning the radiographic-image recording medium with a spot light beam or a light beam having a linearly-shaped cross section.
In the aforementioned reference, the present inventor has proposed a radiographic-image recording medium which can concurrently realize quick response in reading and efficient readout of signal charges. In the proposed radiographic-image recording medium, a first electrode layer, a recording-side photoconductive layer, a charge transportation layer, a reading-side photoconductive layer, and a second electrode layer, where the first electrode layer allows radiation for recording to pass through, the recording-side photoconductive layer becomes conductive when the recording-side photoconductive layer is exposed to the radiation for recording, the charge transport layer behaves substantially as an insulator against latent-image charges and substantially as a conductor for charges having the opposite polarity to the latent-image polarity, the reading-side photoconductive layer exhibits conductivity when the reading photoconductive layer is exposed to an electromagnetic wave for reading (reading electromagnetic wave), and the second photoconductive layer allows the reading electromagnetic wave to pass through. Thus, when the radiographic-image recording medium is irradiated through the first electrode layer with the radiation for recording, a radiographic image is recorded by storing in a charge storage region electric charges the amounts of which correspond to the doses of the radiation. The charge storage region is formed substantially at an interface between the recording-side photoconductive layer and the charge transport layer.
In the above radiographic-image recording medium, the recording-side photoconductive layer directly receives the irradiation. However, the aforementioned reference has further proposed a radiographic-image recording medium in which a wavelength conversion layer is added to the construction described above. The wavelength conversion layer is arranged on the side near the first electrode layer, and contains a fluorescent material which emits visible light when irradiated with the radiation for recording. Thus, the radiation for recording irradiated on the radiographic-image recording medium is converted to visible light in the wavelength conversion layer, and the recording-side photoconductive layer is illuminated with the visible light. When the recording-side photoconductive layer receives the visible light, electric charges are generated in the recording-side photoconductive layer, and stored in the charge storage region. In this case, it is possible to enhance the efficiency in generation of electric charge pairs in the recording-side photoconductive layer, and reduce the dose of radiation which is necessary to expose the recording-side photoconductive layer and the dose of radiation to which a subject (a patient) is exposed.
In the above radiographic-image recording mediums, a DC voltage is applied between the first and second electrode layers so that the first electrode layer is at a negative potential, and the second electrode layer is at a positive potential. At the same time, radiation which has passed through the subject is irradiated on the radiographic-image recording medium through the first electrode layer, so that a radiographic image of the subject is recorded as an electrostatic latent image by storing in the charge storage region negative charges generated in the recording-side photoconductive layer in response to exposure to the radiation which has passed through the first electrode layer.
Then, the application of the above DC voltage is stopped, and the electric charges in the radiographic-image recording medium are rearranged by short-circuiting the first and second electrode layers. Thereafter, when an electromagnetic wave for reading is irradiated on the radiographic-image recording medium through the second electrode layer, the reading photoconductive layer is exposed to the electromagnetic wave which has passed through the second electrode layer, and therefore pairs of electric charges are generated in the reading photoconductive layer. At this time, positive charges in the pairs of electric charges pass through the charge transport layer, and are combined with negative charges stored in the charge storage region, and negative charges in the pairs of electric charges are combined with positive charges stored in the second electrode layer. Thus, discharge occurs, and the latent image is read out by detecting variations in voltages generated between the first and second electrode layers, as variations in currents, by a current detection amplifier or the like.
Incidentally, in many cases, the recording-side photoconductive layers in the radiographic-image recording mediums as described above are made of a-Se (amorphous selenium), since a-Se realizes high dark resistance and response speed. However, for example, in the case where the first electrode layer is formed on a surface of the recording-side photoconductive layer which is formed beforehand, crystallization progresses at the interface between the first electrode layer and the recording-side photoconductive layer made of a-Se, due to heat generated during a vapor deposition process for formation of the first electrode layer or contact between a material of which the first electrode layer is made and the recording-side photoconductive layer made of a-Se. Since the interfacial crystallization increases electric charges which are injected from the first electrode layer when information on a radiographic image is recorded, and the injected electric charges cause noise, the interfacial crystallization lowers the S/N ratio. When the first electrode layer is realized by a transparent oxide film, especially of ITO (indium tin oxide) or IZO (indium zinc oxide), the crystallization remarkably progresses at the interface between the material of which the first electrode layer is made and the recording-side photoconductive layer made of a-Se.
Therefore, in order to prevent the above problem caused by the interfacial crystallization in the recording-side photoconductive layer, the present inventor has proposed an arrangement of a suppression layer between the first electrode layer (through which recording light is applied) and the recording-side photoconductive layer, where the suppression layer is made of an organic polymer which suppresses interfacial crystallization in Japanese Patent Application No. 2001-73376 (which was laid open on Nov. 15, 2002 as Japanese Unexamined Patent Publication No. 2002-329848).
Nevertheless, in the case where the suppression layer made of an organic polymer is used, when operations of recording radiographic images with high radiation doses and reading the radiographic images are repeated, electric charges remain in the suppression layer, and cause problems which include deterioration of sensitivity and remaining of ghost images.